12 AWG copper is 0.0808 in (2.05 mm) across with an area of 6,529 circular mils (3.31 mm²), and measures 1.589 ohms per 1,000 ft at 20°C; 12 AWG aluminum measures 2.604 ohms.
AWG wire gauge chart: diameter, area and DC resistance of copper and aluminum
AWG sizes, solid round wire (ASTM B258 diameters)
AWG / kcmil
Diameter (in)
Diameter (mm)
Area (circular mils)
Area (mm²)
Copper, ohms per 1,000 ft at 20°C (ohm/1000 ft)
Copper, ohms per km at 20°C (ohm/km)
Aluminum, ohms per 1,000 ft at 20°C (ohm/1000 ft)
Aluminum, ohms per km at 20°C (ohm/km)
4/0
0.4600
11.684
211600
107.2
0.04901
0.1608
0.08035
0.2636
3/0
0.4096
10.404
167800
85.01
0.06182
0.2028
0.1013
0.3325
2/0
0.3648
9.266
133100
67.43
0.07793
0.2557
0.1278
0.4192
1/0
0.3249
8.252
105600
53.49
0.09825
0.3223
0.1611
0.5284
1
0.2893
7.348
83690
42.41
0.1239
0.4066
0.2031
0.6665
2
0.2576
6.543
66360
33.62
0.1563
0.5128
0.2562
0.8406
3
0.2294
5.827
52620
26.67
0.1971
0.6466
0.3231
1.06
4
0.2043
5.189
41740
21.15
0.2485
0.8152
0.4073
1.336
5
0.1819
4.620
33090
16.77
0.3134
1.028
0.5138
1.686
6
0.1620
4.115
26240
13.3
0.3952
1.297
0.6478
2.125
7
0.1443
3.665
20820
10.55
0.4981
1.634
0.8165
2.679
8
0.1285
3.264
16510
8.367
0.6281
2.061
1.03
3.378
9
0.1144
2.906
13090
6.631
0.7925
2.6
1.299
4.262
10
0.1019
2.588
10380
5.261
0.9988
3.277
1.637
5.372
11
0.0907
2.304
8226
4.168
1.261
4.136
2.067
6.781
12
0.0808
2.052
6529
3.308
1.589
5.212
2.604
8.544
13
0.0720
1.829
5184
2.627
2.001
6.564
3.28
10.76
14
0.0641
1.628
4109
2.082
2.524
8.281
4.138
13.58
15
0.0571
1.450
3260
1.652
3.181
10.44
5.215
17.11
16
0.0508
1.290
2581
1.308
4.019
13.19
6.588
21.62
17
0.0453
1.151
2052
1.04
5.054
16.58
8.285
27.18
18
0.0403
1.024
1624
0.8229
6.386
20.95
10.47
34.35
19
0.0359
0.912
1289
0.653
8.047
26.4
13.19
43.28
20
0.0320
0.813
1024
0.5189
10.13
33.23
16.6
54.47
21
0.0285
0.724
812.3
0.4116
12.77
41.89
20.93
68.67
22
0.0253
0.643
640.1
0.3243
16.2
53.16
26.56
87.15
23
0.0226
0.574
510.8
0.2588
20.31
66.62
33.29
109.2
24
0.0201
0.511
404
0.2047
25.67
84.22
42.08
138.1
25
0.0179
0.455
320.4
0.1624
32.37
106.2
53.06
174.1
26
0.0159
0.404
252.8
0.1281
41.02
134.6
67.25
220.6
27
0.0142
0.361
201.6
0.1022
51.43
168.7
84.32
276.6
28
0.0126
0.320
158.8
0.08044
65.33
214.3
107.1
351.4
29
0.0113
0.287
127.7
0.0647
81.22
266.5
133.2
436.8
30
0.0100
0.254
100
0.05067
103.7
340.3
170
557.8
31
0.0089
0.226
79.21
0.04014
130.9
429.6
214.6
704.2
32
0.0080
0.203
64
0.03243
162.1
531.7
265.7
871.6
33
0.0071
0.180
50.41
0.02554
205.7
675
337.3
1107
34
0.0063
0.160
39.69
0.02011
261.3
857.3
428.4
1405
35
0.0056
0.142
31.36
0.01589
330.7
1085
542.2
1779
36
0.0050
0.127
25
0.01267
414.8
1361
680.1
2231
37
0.0045
0.114
20.25
0.01026
512.2
1680
839.6
2755
38
0.0040
0.102
16
0.008107
648.2
2127
1063
3486
39
0.0035
0.089
12.25
0.006207
846.6
2778
1388
4554
40
0.0031
0.079
9.61
0.004869
1079
3541
1769
5804
kcmil sizes, Class B concentric stranded copper and aluminum
AWG / kcmil
Diameter (in)
Diameter (mm)
Stranded OD, Class B (in)
Stranded OD, Class B (mm)
Area (circular mils)
Area (mm²)
Copper, ohms per 1,000 ft at 20°C (ohm/1000 ft)
Copper, ohms per km at 20°C (ohm/km)
Aluminum, ohms per 1,000 ft at 20°C (ohm/1000 ft)
Aluminum, ohms per km at 20°C (ohm/km)
250 kcmil
0.5000
12.700
0.575
14.61
250000
126.7
0.04231
0.1388
0.06937
0.2276
300 kcmil
0.5477
13.912
0.630
16.00
300000
152
0.03526
0.1157
0.05781
0.1897
350 kcmil
0.5916
15.027
0.681
17.30
350000
177.3
0.03022
0.09916
0.04955
0.1626
400 kcmil
0.6325
16.064
0.728
18.49
400000
202.7
0.02645
0.08677
0.04336
0.1422
500 kcmil
0.7071
17.961
0.813
20.65
500000
253.4
0.02116
0.06941
0.03468
0.1138
Diameters are ASTM B258 nominal values: the AWG formula rounded to the nearest 0.1 mil (0.0001 in), as NBS Handbook 100 describes. Areas and resistances are computed from those rounded diameters, the same way NBS Handbook 100 does it.
Resistance is DC resistance of solid annealed copper at 100% IACS (NBS Handbook 100) and of aluminum 1350-H19 (EC-H19) at 61.0% IACS (NBS Handbook 109), both at 20°C (68°F). For another temperature T in °C multiply by 1 + 0.00393 (T - 20) for copper or 1 + 0.00403 (T - 20) for aluminum.
Stranded wire of the same AWG has the same nominal area but a larger diameter, and its resistance is higher because the strands spiral: NBS Handbook 100 uses 2% for sizes up to 2,000,000 circular mils. The kcmil rows are Class B concentric stranded (37 wires). Their Diameter column is the solid-equivalent diameter (the square root of the area in circular mils), the Stranded OD column is the diameter over the strands from NBS Handbook 100 Table 17, and the resistance includes that 2%.
MWS Wire Industries' EC aluminum table reads about 0.3% lower than this table (12 AWG: 2.596 against 2.604 ohms per 1,000 ft at 20°C); this table uses the 61.0% IACS value from NBS Handbook 109.
This table gives physical properties only. It does not give ampacity, the current a conductor can carry continuously under its conditions of use without exceeding its temperature rating; get that from the electrical code adopted where you work and the wire maker's data.
Cross-checked against National Bureau of Standards (now NIST), Handbook 100, Copper Wire Tables (1966), Tables 1, 5, 10 and 17 and MWS Wire Industries, Copper Magnet Wire Data (bare copper, 6 to 56 AWG) and Southwire, SPEC 80150 Bare Copper Wire and Cable, Solid and Stranded (rev. Jan 26, 2026) and MWS Wire Industries, Aluminum and Aluminum Alloys: EC (1350) Aluminum Wire Data and Southwire, AAC All Aluminum Conductor, Bare (ASTM B230/B231 1350-H19) specification: Copper: all 44 AWG rows (4/0 to 40) match Table 5 (diameter, circular mils, ohms per 1,000 ft) and Table 10 (mm², ohms per km) to the handbook's printed precision (it prints 20.25 and 12.25 cmil for 37 and 39 AWG as 20.2 and 12.2, and 1 AWG at 0.40655 ohms per km rounds to its 0.4065). One cell in the scanned text layer reads 1200 cmil for 19 AWG; the printed diameter, 35.9 mils, gives 1289, which MWS also lists, so it is a scan misread. kcmil: our stranded values corrected to 25°C match Table 17 for all five sizes.; Copper: all 35 rows from 6 to 40 AWG match MWS nominal bare diameter, circular mils and nominal resistance within 0.06%.; Copper: 14 solid sizes (14 to 1 AWG) match Southwire's soft-drawn diameter, area and 20°C resistance, except 13 AWG, where Southwire lists 2.003 against our 2.001 (MWS also lists 2.001). Stranded 4/0 to 1/0 and 250 to 500 kcmil equal our solid values times the 2% stranding increment (e.g. 250 kcmil 0.04231), and Class B diameters 575, 630, 681 match; Southwire lists 500 kcmil at 814 mils against Table 17's 813. Southwire's 3/0 19-wire row is marked as interpolated and was not used.; Aluminum: all 31 rows from 10 to 40 AWG run 0.31 to 0.37% above MWS. MWS values imply 61.2% IACS; this table uses the 61.0% value from NBS Handbook 109, the more conservative (higher) resistance.; Aluminum: 6 AWG to 4/0 and 250 to 500 kcmil stranded 1350-H19 resistance at 20°C runs 0.1 to 0.5% below our values times 1.02, consistent with Southwire's three-figure rounding and a 61.2% IACS basis..
How AWG sizes are defined
American Wire Gauge fixes two sizes and spaces the rest between them in a geometric progression: 36 AWG is 0.0050 in and 4/0 (0000) is 0.4600 in, with 38 sizes in between. ASTM B258 prescribes the nominal diameters. As NBS Handbook 100 describes it, each one is calculated as d = 0.005 in x 92(36 - n)/39 and rounded to the nearest 0.1 mil (0.0001 in). NBS notes that these rounded diameters are the ones used commercially, and every area and resistance in the chart is computed from them.
Because each step is the same ratio (1.1229 in diameter), NBS Handbook 100 lists a few shortcuts that hold across the whole range:
Every 3 gauges doubles the area and halves the resistance: 10 AWG is 10,380 circular mils, 13 AWG is 5,184.
Every 6 gauges doubles the diameter: 12 AWG is 0.0808 in, 18 AWG is 0.0403 in.
Every 10 gauges multiplies the area by about 10: 20 AWG is 1,024 circular mils, 10 AWG is 10,380.
NBS Handbook 100 notes that conductors larger than 4/0 are generally named by their area in circular mils. In kcmil (thousands of circular mils), a 250 kcmil conductor has 250,000 circular mils, or 126.7 mm².
Reading the columns
Diameter is the bare solid wire, in inches and millimeters. Stranded wire of the same gauge has the same metal area but a larger outside diameter. The kcmil rows give two diameters: a solid rod of the same area, and the outside of a standard concentric (ASTM Class B) 37-wire stranded conductor from NBS Handbook 100, Table 17. 250 kcmil is 0.500 in as a solid rod and 0.575 in stranded.
Area in circular mils is the diameter in mils squared: 12 AWG is 80.8 x 80.8 = 6,529. It is the CM in the voltage drop formula.
Area in mm² is the true cross-section. One circular mil is 0.0005067 mm². See the AWG to mm² converter for metric cable sizes.
Resistance is DC resistance at 20°C (68°F), per 1,000 ft and per km, for annealed copper at the international standard (10.371 ohm-cmil/ft, NBS Handbook 100) and for aluminum 1350-H19 (EC-H19 in NBS Handbook 109) at 61.0% of copper's conductivity (17.002 ohm-cmil/ft). Resistance of a run is the per-1,000-ft value times the total wire length in thousands of feet: 200 ft of 12 AWG copper (100 ft out and back) is 0.318 ohms.
Copper vs aluminum
At 61.0% of copper's conductivity, aluminum 1350 has 1.64 times the resistance of copper at the same size. NBS Handbook 100 gives 1.2610 as the area ratio from one gauge to the next, so going up two gauges adds about 59% more area, and aluminum two sizes larger comes close to copper: 4 AWG aluminum measures 0.4073 ohms per 1,000 ft against 0.3952 for 6 AWG copper, and 2 AWG aluminum measures 0.2562 against 0.2485 for 4 AWG copper.
Manufacturer tables can read slightly lower. MWS Wire Industries, which identifies EC aluminum as alloy 1350, lists 12 AWG at 2.596 ohms per 1,000 ft at 20°C, about 0.3% below this chart's 2.604. The chart uses the 61.0% value from NBS Handbook 109, the higher resistance. Stranded conductors measure about 2% more than solid ones of the same size, because the strands spiral: NBS Handbook 100 uses 2% up to 2,000,000 circular mils and up to 5% above that. Southwire lists stranded 4/0 soft copper at 0.04999 ohms per 1,000 ft at 20°C; the chart's solid 4/0 is 0.04901, 2% less.
Temperature
NBS Handbooks 100 and 109 give resistance rising by about 0.393% per °C for copper and 0.403% per °C for aluminum, measured from 20°C. Multiply the chart value by 1 + 0.00393 x (T - 20) for copper. At 75°C, the temperature IAEI gives for the published voltage drop constants, copper reads 1.216 times the chart: 12 AWG goes from 1.589 to 1.932 ohms per 1,000 ft (NBS Handbook 100 prints 1.93 at 75°C). That rise, plus the 2% allowance for stranding, is why the voltage drop constant for copper is 12.9 rather than 10.4.
Why ampacity is not on this page
Cerrowire defines ampacity as the maximum current a conductor can carry continuously under specific operating conditions without exceeding its temperature limit. It is not a property of the metal alone. Cerrowire's published ampacities are based on the National Electrical Code (NFPA 70), and it notes that they do not include the temperature correction or ampacity adjustments that may also be required. FieldCharts does not reproduce the NEC ampacity tables.
To find ampacity, use the edition of the code adopted where you work; NFPA offers free read-only access to its codes online, though not every edition is available that way. Some manufacturers list ratings for their own products, and those ratings come with conditions: Southwire's bare copper specification gives its ampacities at a 75°C conductor temperature, 25°C ambient and 2 ft per second of wind in sun. For equipment, check the installation manual. Cerrowire also points to the local building inspector or a qualified electrician.